A nickel plated Colt Detective Special is carried by Chicago patrolman George Stone (Andy Garcia) as his backup gun, which he tosses to Ness during the train station shootout.
Colt Official Police
Officer Stone (Andy Garcia) uses a Colt Official Police while on the police academy firing range. The court officer is later seen with one that he lends to Ness for him to chase Nitti down. What is interesting about Ness's gun is that this particular version of the Official Police is actually a Post-War gun (meaning it wasn't made until after 1950) whereas this movie takes place in the 1930s. The ramped front sight, flat hammer and longer ejection rod cap are changes that were made after 1950. Pre-War guns would have a halfmoon front sight and shorter ejector rod cap.
Some of these pictures look like Colt M1911A1s, while others are Star Model Bs--User: Colt Revolver Fan
M1911A1
Treasury agent Elliot Ness (Kevin Costner) uses an M1911A1 throughout the film in scenes in which he is not firing his weapon. Treasury accountant Oscar Wallace (Charles Martin Smith) keeps one in a shoulder holster. Several of Capone's (Robert De Niro) mobsters carry them.
Star Model B
Treasury Agent Elliot Ness (Kevin Costner) carries a Star Model B as his sidearm in the film, standing in for the M1911A1 in scenes in which he fires it. A gangster who holds the bookkeeper (Jack Kehoe) from Ness and Stone in the train station holds a Star Model B to the bookkeeper's head. Frank Nitti (Billy Drago) carries a nickel-plated Star Model B in court, which Ness uses as an excuse to kick him out.
Smith & Wesson Model 10
A Smith & Wesson Model 10 is carried by Stone (Andy Garcia). He uses it to kill the gangster who is holding the bookeeper hostage at the end of the shootout in the train depot. Malone (Sean Connery) also carries one, which he infamously uses to shoot an already dead gangster. Many other Chicago policemen carry Model 10s.
Thompson M1928
Many gangsters and policemen use the Thompson M1928 during the film. Stone and Malone use them to great effect during the Canadian shootout. Frank Nitti notably uses one to fatally wound Malone outside his apartment.
According to the Ellis Props & Graphics NFA weapons list available on Long Mountain Outfitters' web site, there was a mix of M1921s and M1928 Thompsons used in the film. -MT2008
Webley MK IV
The leader of the Royal Canadian Mounted Police is seen firing a Webley MK IV into the air when he prematurely launchs the assualt on Capone's men at the border.
Colt New Service
One of the Royal Canadian Mounted Police officers is seen firing a Colt New Service while on horse back.
The Chicago Police Department uses Winchester Model 1894 rifles. A number of Winchester '94s can be seen in a gun rack at a police station.
Winchester Model 1897
The Untouchables use Winchester Model 1897 shotguns on several occasions, such as the post office raid.
Winchester Model 1912
A Winchester Model 1912 is used by Elliot Ness (Kevin Costner) during the shootout in the train station. These are also used during the shootout on the Canadian/U.S. border.
Rossi Overland sawed-off shotgun
An 12 Gauge Rossi Overland sawed-off shotgun is kept by Malone (Sean Connery) in his apartment as a "home defense" weapon. The Rossi Overland sawed-off shotgun was built by the late gunsmith Branko Wohlfahrt for Ellis Mercantile.
Mk 2 hand grenade
Ness launches a Mk 2 hand grenade over their Canadian cabin to force a Capone gangster to run inside.